In the automobile industry, because of environmental issues, electric and the hybrid cars having an internal combustion engine combined with an electric motor driven by electricity supplied from a battery have been developed, and are now in the marketplace and in practical use. A nickel metal hydride rechargeable battery, a lithium ion rechargeable battery, or an electric double layer capacitor are used as batteries and the lithium ion capacitor has attracted attention recently.
A battery which is installed in the automobiles as a power supply means should have both a high output property and a high energy density. The lithium ion capacitor has a high output property which is not accomplished by a lithium ion battery and has a high energy density property which is not accomplished by an electric double layer capacitor. Therefore, the lithium ion capacitor has attracted attention as a future battery.
A positive electrode of the lithium ion capacitor consists of an active carbon which is used in the electric double layer capacitor and a negative electrode consisting of a carbon material which is capable of storing and releasing lithium ions.
In the lithium ion battery, at an initial lithium ion charging, a SEI film is formed on the negative electrode. Therefore, the initial charge-discharge efficiency never reaches 100%. As the lithium ion capacitor works under the same principle as the lithium ion battery, it is natural that an initial charge-discharge efficiency of the lithium ion capacitor is less than 100%.
In the lithium ion capacitor, because lithium ions are pre-doped to the carbon materials of the negative electrode, the capacity of the negative electrode is high and also the electric potential of the negative electrode becomes low. Consequently, an energy density of the lithium ion capacitor becomes 4 times as much as a conventional electric double layer capacitor and, further, test results of a high temperature load test and a cycle test are satisfactory and a long term reliability of the products is expected.
In a patent publication of document 1 (JP2008-150270), we disclosed porous carboneous powder for a negative electrode of a lithium ion capacitor, which is a composite carbon material of a carbon black and a carbonaceous precursor.
A development of a higher energy density lithium ion capacitor is expected. A proposed effective solution to the goal of a higher energy density capacitor is to reduce the volume of a cell of the lithium ion capacitor by making the thickness of the layer of the negative active material thinner while keeping the energy density unchanged.
For the purpose of keeping the energy density unchanged when the cell volume is reduced, it is necessary to increase the amount of doped lithium ions, which is interpreted as to increase the initial charge capacity per unit weight of the negative electrode when a lithium metal of the negative active material is used as a counter electrode.
However, the negative electrode active material disclosed in patent document 1 does not exhibit a desired performance and a good result in an initial charge capacity per unit weight of the negative electrode and the purpose of increasing the energy density of the lithium ion capacitor is not yet achieved.